3  5 


ILLINOIS  STATE 

LABORATORY 
OF  NATURAL  HISTORY 

LIBRARY 


•e+4+4+4+444+4+4+t+4+44 


X  I  E>  HAR.Y 

OF   THE 

U  N  I  VERSITY 

Of    ILLINOIS 

^URAL  HISTORY-  SURVE 

550.5 

FI 
v.3,cop.CL 


REMOTE  gf  L 


LIBRARY 
UNIVERSITY  OF  ILLINOIS 

URBANA 


Field  Columbian  Museum. 

Publication  112. 

Geological  Series.  Vol.  Ill,  No.  4. 


ZOISITE  FROM 
LOWER  CALIFORNIA 


BY 


Oliver  Cummings  Farrington,  Ph.  D. 
Curator,  Department  of  Geology. 


Chicago,  U.  S.  A. 
May   15,  1906. 


ZOiSITE  FROM  LOWER  CALIFORNIA 


BY  OLIVER  CUMMINGS  FARRINGTON 

While  at  San  Diego,  California,  in  the  spring  of  1905,  the  writer 
obtained  from  Mr.  Ernest  Riall  of  that  city  several  specimens  of  a 
radiated  mineral,  collected  by  Mr.  Riall  at  the  Trace  mine,  in  the 
Juarez  District  of  Lower  California.  This  locality,  according  to  Mr. 
Riall,  is  situated  sixty  miles  south  of  the  international  boundary.  The 
accompanying  cut  shows  the  appearance  of  a  typical  specimen  of  the 
mineral.  It  occurs  as  divergent  groups  of  long,  prismatic  crystals  irreg- 
ularly penetrating  a  matrix  of  a  white,  granular  mineral.  The  length 
of  the  crystal  groups  varies  from  one  to  three  inches.  Their  form 
is  essentially  conical,  the  angle  of  the  cone  being  about  io°.  In  coloring 
the  cones  are  pink  peripherally,  pass  interiorly  into  nearly  colorless 
and  at  the  center  are  brownish-gray.  Their  constitution  of  numerous 
individual  crystals  is  shown  by  elongated  brilliant  surfaces  into  which 
they  readily  separate  longitudinally,  but  transversely  the  cones  break 
as  units.  The  cones  as  a  whole  are  translucent,  but  small  fragments 
are  transparent.  No  terminal  planes  can  be  observed  on  any  of  the 
crystals.  The  longitudinal  fragments  show  roughly  prismatic  bound- 
aries, but  it  was  found  impossible,  with  a  reflecting  goniometer,  to  ob- 
tain satisfactory  measurements  of  the  prismatic  angles,  since  numer- 
ous longitudinal  striations  produce  long  series  of  reflections.  Besides 
the  striated  planes,  others  not  striated  appear  to  be  cleavage  planes 
to  the  brachypinacoid.  The  longitudinal  fragments  are  colorless  and 
transparent  and  show  in  polarized  light  extinction  parallel  to  the  long 
axis.  No  pleochroism  is  observable.  The  character  of  the  double 
refraction  is  positive.  In  convergent  light  the  emergence  of  an  optic 
axis  may  be  seen  on  such  fragments.  Sections  perpendicular  to  the  axis 
of  the  cone  are  colorless  and  show  no  pleochroism.  Numerous  cleavage 
cracks  making  angles  of  530  with  each  other  penetrate  such  sections. 
Between  crossed  nicols  a  polysynthetic  twinning  structure  is  seen  to 
characterize  the  whole,  the  field  being  filled  with  lamellae  in  parallel 
position.  These  lamellae  divide  into  two  groups  as  regards  width, 
the  broader  being  from  .1  to  .07  mm.  and  the  narrower  from  .025  to 
.012  mm.     The  broad  and  narrow  lamellae  alternate.     The  direction 

55 


56  Field  Columbian  Museum — Geology,  Vol.  III. 

of  the  lamellae  is  such  as  to  bisect  the  above-noted  cleavage  angle 
of  530.  Single  cleavage  cracks  ||  with  the  lamellae  are  also  occa- 
sionally seen. 

The  interpretation  of  this  structure  is  difficult,  but  the  following 
may  be  suggested:  The  cleavage  cracks  at  angles  of  53 °  are  those  of 
prisms  of  which  the  crystal  groups  are  composed.  These  prisms  have 
the  symbol  540,  corresponding  to  an  angle  of  520  44'.  This  is  a  new 
form  for  zoisite.  The  twins  of  which  these  prisms  are  made  up  are 
formed  on  c  as  the  twinning  axis  and  the  twinning  plane  is  some 
highly  inclined  brachy-dome  such  as  e  (061). 

The  lustre  of  fragments  of  the  mineral  is  vitreous  and  the  frac- 
ture sub-conchoidal.  Hardness  6.5  and  specific  gravity,  determined 
with  a  chemical  balance,  3.32.  The  mineral  fuses  B.  B.  at  3  with  in- 
tumescence, to  a  brownish  enamel  and  is  only  slightly  attacked  by 
hydrochloric  acid.  Qualitative  tests  showed  it  to  be  essentially  a 
hydrous  calcium  aluminum  silicate,  from  which  the  water  could  be 
driven  off  only  by  strong  ignition.  Quantitative  analysis  by  Mr.  H. 
W.  Nichols  gave  the  following  result: 

Ratio 

SiO,  38.15                                    3.02 

ALP,  29.50  \ 

FeA  4.60  f  I51 

MuO  0.55 

CaO  22.71  / 

MgO  0.63  \  2°5 

'  H20  3.76                                 1. 00 

K*°    I  tr 

Na,0  f  tn 


99.90 

These  ratios  lead  to  the  formula  H4  Ca4  Al6  Si6  Oa,,  which  is 
that  usually  accepted  for  zoisite  with  the  addition  of  one  molecule 
of  water.  For  the  determination  of  the  water  both  of  Penfield's 
methods*  were  employed.  By  the  first  method,  that  of  heating  in  a 
blast  lamp,  1.81%  of  water  was  obtained.  The  mineral  did  not  fuse. 
By  the  second  method,  which  consists  in  heating  the  tube  containing  the 
assay  in  an  oven  of  fire-brick  lined  with  charcoal,  an  additional  percent- 
age of  water  amounting  to  1.95%  was  obtained.  Under  this  treatment 
the  mineral  fused  completely.  The  close  similarity  between  the  per- 
centages of  water  obtained  by  the  two  methods,  each  corresponding  to 
one  molecule,  suggests  that  the  molecules  may  be  differently  com- 
bined.    Thus  one  may  be  united  with  aluminum  and  the  other  with 

*  Amer.  Jour.  Sci.  1894,  3rd  ser.  Vol.  XLVIII,  pp.  30-37 


May,  1906.      Lower  California  Zoisite — Farrington.  57 

calcium.  No  further  investigation  was  made  of  this  point,  however. 
Although  the  formula  of  zoisite  is  usually  considered  to  be  Hs  Ca4  Al4 
Si„  0„,  other  analysts  have  obtained  percentages  which  indicate 
that  an  additional  molecule  of  water  is  present.  This  is  true,  for 
instance,  of  the  analyses  of  zoisite  from  Fuschthal  and  Traversella 
quoted  by  Dana.*  The  high  temperature  required  to  drive  off  the  water 
from  the  Lower  California  mineral  seems  to  preclude  the  possibility 
of  its  being  present  as  the  result  of  alteration,  as  might  otherwise 
be  assumed.  The  amount  of  iron  in  the  zoisite  shown  by  the  analysis 
is  high  for  this  mineral  and  approximates  that  afforded  by  epidote. 

The  mineral  with  which  the  zoisite  is  associated  is,  as  stated, 
white  and  granular.  In  cavities  it  exhibits  minute  imperfect  crystals 
which  have  a  distinct,  pearly  luster  when  fractured.  The  blowpipe 
and  other  characters  of  this  mineral  indicate  it  to  be  prehnite  and  a  com- 
parison with  fragments  kindly  furnished  by  Dr.  W.  T.  Schaller  leaves 
little  doubt  that  it  is  the  same  mineral  analyzed  f  by  him  and  found 
to  be  prehnite.  Its  association  with  zoisite  is  of  interest  owing  to  the 
similarity  in  composition  of  the  two  minerals.  The  prehnite  seems 
generally  to  furnish  a  matrix  which  the  zoisite  penetrates,  but  occa- 
sionally it  coats  the  zoisite  groups  in  such  a  way  as  to  suggest  that  it 
is  an  alteration  product  of  the  latter.  The  unusual  features  of  the 
zoisite  seem  to  be  therefore,  its  radiating  habit,  its  high  content  of 
water  and  iron  and  its  association  with  prehnite. 

Through  the  kindness  of  Prof.  L.  P.  Gratacap  of  the  American 
Museum  of  Natural  History,  ths  writer  was  permitted  to  study  two 
specimens  of  zoisite  in  the  collection  of  that  institution  which  were 
undoubtedly  from  the  same  locality  as  the  above.  They  have  the 
more  usual  ash-gray  color  of  zoisite  and  the  grouping  of  the  crystals  into 
cones  is  only  partial.  For  the  most  part  the  crystals  occur  in  hemi- 
spherical cavities  which  were,  in  the  specimens  studied,  about  three 
inches  in  diameter.  The  crystals  interlace  these  cavities  with  great 
variations  of  size  and  direction.  Many  of  the  crystals  are  quite 
minute.  All  are  from  acicular  to  bladed  in  habit.  Although  some 
crystals  have  free  terminations,  no  end  faces  could  be  discerned. 
These  specimens  show  that  grouping  into  cones  is  not  constant  for  the 
zoisite  from  this  locality  but  its  occurrence  at  all  is  noteworthy . 

*  System  of  Mineralogy,  6th  ed.,  p.  514. 
tBull.    U.S.Geol.  Survey  No.  262,  p. 128 


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